1. Field of the Invention
The invention relates also to a miniaturized deflectable membrane separating a lower chamber containing a clean pressure transmission medium from an upper chamber receiving through an aperture of said upper chamber, a fluid, the pressure of which is to be measured.
The invention relates also to a shell of a pressure sensor containing the upper and lower chamber and the membrane.
2. Description of the Prior Art
A pressure sensor module generally comprises a sensor shell having an aperture for receiving a fluid, the pressure of which is to be measured by the sensor. The shell comprises a separating membrane which can be deflected when a positive or negative pressure is applied by the fluid present at the aperture of the shell. In order to minimize unwanted pressure effects, ideally, such a separator must be “transparent” to pressure. Its stiffness must then be as low as possible. For a given material, the stiffness of the membrane is increasing when its thickness increases and decreases when its area increases. Low stiffness is obtained with large and/or thin membranes. Usually, for reliability and robustness raisons, these membranes are metallic and are thicker than 50 micrometers.
The separating membrane tightly separates an inner closed volume of a lower chamber of the shell from an open volume of an upper chamber of the shell having the aperture. The lower chamber of the shell contains a pressure transmitting medium and a measuring cell. The pressure of the fluid present in the open volume of the shell is then indirectly applied to the measure cell, by means of the membrane and of the transmitting medium, for instance silicon oil. Because of the separating membrane the cell is immerged in the pressure transmitting medium and is then protected from corrosive fluids, the pressure of which is to be measured. The measuring cell is made of two half cents bonded to one another. A hollow part inside the measuring cell allows for a deflection of a diaphragm of the cell. Resistors forming a bridge are mechanically linked, for instance bonded or deposited on a surface of the diaphragm of the measuring cell. When the diaphragm of the cell having the resistors linked on it is deflected by the pressure of the transmitting medium, the resistors are bent. The bending results in a modification of the resistance of the resistors. The resistors may be made of thin film which are bonded or deposited onto the diaphragm. A kind of thin film which is used for the purpose of making resistors, in particular resistors to be used in pressure cells is made of a mix of tantalum (Ta) grains in a nitride of tantalum (NTa).
Bridge excitation in the form of a voltage or current is applied across two opposite nodes of the bridge. These nodes are usually referred to as excitation inputs or bridge drive inputs. The bridge inputs are connected through tight connections to external connections of the measuring cell and then on external connections of the shell.
To get an easily deflectable membrane one uses generally large membranes (more than a centimetre in diameter) or a kind of bellows made by a plurality of walls. The walls of the membrane have together the form of the wall of an accordion. An aperture of the accordion like wall is turned toward the lower chamber so that an inner volume of the bellow is filled with the clean transmitting medium. A drawback of such a construction for the membrane separating the lower chamber from the upper chamber is that necessarily, it can become very long or too large for miniaturized system. In addition, the inner volume of the bellow is great, and so the quantity of the transmitting medium, for instance silicon oil is great too. That makes the signal transmitted from the cell to vary with the variations of the volume of the transmitting medium due to variations of the temperature of the transmitting medium.